This invention relates to power transmission systems and more particularly to a continuously variable transmission (CVT) of the hydraulic type.
A CVT system allows an engine to operate at its optimal fuel consumption point. The standard transmission is capable of a predetermined number of gear ratios as the torque demand for the vehicle is high or low, generally three or four gear ratios in automatic transmissions up to five in manual transmissions. A CVT system, on the other hand, transfers power in a continuously changeable power ratio between the high and low torque demands. The speed ratio in a CVT can be varied in exact accordance with torque demand rather than varied approximately as in standard transmission systems.
Interest is increasing in the efficiency of operation of motor vehicles. The long history of CVT research, however, has not provided a fully practical transmission for motor vehicles except for certain compact and subcompact cars and for small vehicles, such as snowmobiles and lawnmowers. The CVT provides better fuel economy, is lighter and has a lower manufacturing cost than the standard three-speed automobile transmission. The reason for the latter advantage is that the CVT has about a third fewer parts than the standard transmission.
CVT systems can be classified as belt and pulley, traction drive, and fluid or hydraulic power. The hydraulic power CVT generally uses a pump as a part of the system. This latter type of CVT is used in certain types of off-road vehicles but, because of high cost, weight, limited range of efficiency, and noisy operation, it is not used for common motor vehicles.
Most CVT systems, including the hydraulic CVT, are adapted for use with the internal combustion engine, that is, with a power shaft that varies in rotational speed. They are not generally adapted for use with constant speed power shafts, such as those driven by electric motors.